Boxing gives you striking ability, this alone does not win a fight. Somebody could be an amazing boxer in a ring, that doesn't mean this person has the mentality to hit someone without gloves.

Of course having a striking ability gives you an advantage over any average Joe. But street fights have many, many different factors to take into account; how you're being attacked (grabbed, held on to, thrown to the ground etc), how many people are attacking you, weapons, where you are etc will affect how you fight.

Overall boxing can be useful if you get lucky and land a punch or two, if not it's useless. Being a boxer does not make you a fighter.